Hi sjp1966

For column A, Select column or range of cells > Conditional formatting > New Rule etc.

Condition 1. Greater then equal to C1: =AND($A1<>"",$A1>=$C1) Format as required.
Condition 2. Less the C1: =AND($A1<>"",$A1<$C1) Format as required.

Same for Column B adjust cell references as required